New SW Pacific tectonic model: Cyclical intraoceanic magmatic arc construction and near‐coeval emplacement along the Australia‐Pacific margin in the Cenozoic

Abstract

[1] Various reconstructions of the SW Pacific for the Late Cretaceous and Cenozoic suggest that northeast dipping subduction began in the South Loyalty Basin (SLB) at 55-50 Ma and that subsequent closure of the SLB resulted in the diachronous emplacement of Cretaceous-Paleocene ophiolitic nappes onto the Norfolk Ridge in New Caledonia at 40-34 Ma and in Northland, New Zealand, around 24-21 Ma. A fundamental problem with these models is that they do not account for the fact that NE dipping subduction had already been established offshore Papua New Guinea by at least 65-60 Ma which resulted in the emplacement of the Papuan Ultramafic Belt (PUB) ophiolite at 59-58 Ma. A second issue is that the reconstructions are based largely upon unfounded assumptions as to the age and nature of the basement beneath the Loyalty arc and Three Kings Ridge. Finally, reconstructions of the Northland region are based upon the erroneous assumption that the age of the majority of the igneous component comprising the Northland allochthon is Late Cretaceous-Paleocene, when in fact it is Oligocene. A new model is presented whereby the PUB, New Caledonia, and Northland ophiolites formed and were emplaced in a cyclical fashion above an extensive NE dipping Cenozoic intraoceanic arc system which diachronously propagated (N-S) along the entire eastern margin of the Australian Plate. These ''infant arc'' ophiolites represent fragments of suprasubduction zone lithosphere (SSZL) generated in the earliest stages of magmatic arc formation that were emplaced shortly after (<20 m.y.) as a result of forearc-Australian Plate collision. Subduction inception was the result of subsidence of older MORB-like lithosphere generated within an extensive ''back arc basin'' to the east of the Norfolk Ridge during the earliest stages of SLB formation above a southwest dipping Pacific Plate. During emplacement of each ophiolite, a crustal fragment of the older lithosphere was scraped off the NE dipping slab and subsequently back-thrust beneath each ophiolite during its emplacement.

Different styles of modern and ancient non-collisional orogens and implications for crustal growth: a Gondwanaland perspective

“…Syntheses of this history have been provided by several authors (e.g., Hall 2002;Crawford et al 2003a;Sdrolias et al 2003;Schellart et al 2006;Whattam et al 2008). This rollback is inferred to have occurred in several stages, leading to continental ribbons being rifted sequentially from the Australian mainland, and separated by major sedimentary basins.…”

From continent to intra-oceanic arc: Zircon xenocrysts record the crustal evolution of the Solomon island arc

“…3; Fig. DR2) resulted from microcontinental fragments underthrusting shortlived east-northeast-dipping subduction zones (Whattam et al, 2008). The Eocene onset of ophiolite deformation on Guadalcanal (Hackman, 1980) correlates with the termination of spreading in the Coral Sea and of northeast subduction at the Pocklington Trough (Fig.…”

“…Spreading in the Coral Sea at 62-52 Ma was induced by a short-lived episode of northeast-dipping subduction at the Pocklington Trough (Whattam et al, 2008; Fig. 3).…”

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“…General agreement exists that the modern Tonga-Kermadec system evolved from a boundary lying near the Norfolk Ridge and New Caledonia in the middle Eocene. However, ideas about the exact geometry of that boundary or the nature of Cretaceous to middle Eocene plate boundaries northeast of Norfolk Ridge (Whattam et al, 2008) vary because the region has either been subducted or deformed and intruded and it is sparsely sampled.…”